Side branch resonators have been used for many years to reduce radiated induction noise in automobile engine compartments. In one common application, a resonator having a cavity includes a neck interconnecting the cavity to the air intake duct of the engine induction system. The compartment and hence cavity are typically tube or rectangular shaped and there may be one or more necks (see, for example, U.S. Pat. No. 6,609,489). The parameters of cavity, neck length and neck diameter dictate at what frequency the resonator will resonate. The resonating frequency is chosen to match the frequency of the induction noise. Thus, when designing a resonator, the engineer will choose the cavity and neck diameter and length to achieve a resonating frequency that will match and cancel the frequency of the induction noise it is desired to attenuate.
The strength of the resonator is proportional to the square root of the cavity volume for a constant neck size. Thus, strong resonators require a large resonator cavity, however, large cavity size may not be feasible due to space constraints in the engine compartment. In other words, the available space dictates how large the engineer can make the cavity in terms of length, width and depth. Another potential problem is that making one dimension much larger than the other two will cause the resonator cavity to exhibit plane wave behavior and the resonator will thus not resonate at the predicted frequency. The engineer is thus forced to reduce this dimension size until the plane wave behavior ceases, however, this reduces the resonator strength as well. It will thus be appreciated that resonator design has been limited by space availability and attempts to increase resonator strength through an increase in linear dimensions of the cavity are typically futile.
There therefore exists a need for an improved resonator and method for reducing induction noise emanating from an engine that provides strong attenuation while occupying a small space in the engine compartment.